The present invention relates to meters which derive a measurement by vibrating a metering tube, particularly Coriolis-type flow meters, such as are described in U.S. Pat. No. 4,422,338, U.S. Pat. No. 5,423,221, U.S. Pat. No. 4,856,346, U.S. Pat. No. 5,394,758, U.S. Pat. No. 4,192,184 and U.S. Re. Pat. No. 31,450, the disclosures of each of which are herein incorporated by reference. The invention is also applicable to other meters, for example densitometers which operate by vibrating a metering tube.
Coriolis meters may be used to obtain a measure of mass flow rate, from the phase difference between sensor outputs and also a measure of density, from the resonant frequency. Pursuant to the invention, we have found that density measurements in conventional meters may be inaccurate, particularly when the stress in the metering tube varies, for example when the meter is subjected to a change in temperature, either ambient temperature or temperature of process fluid. The problem is particularly acute in straight, or nearly straight, tube meters, where we have found that uncompensated errors of tens of percent and even compensated errors of several percent may arise in the event of a change in process fluid temperature in a meter which is intended to have an accuracy of the order of 0.1%.
Our investigations have shown that, at least in the case of a straight tube meter, the errors arise primarily due to changes in tension in the metering tube. It is known that frequency of vibration is dependent on tension in the tube (as well as fluid density) and it is known to take a single measurement of tension and store this. However as mentioned, the tension is liable to change, particularly with temperature fluctuations, and also with aging of the tube.
To enable tension measurements to be made in situ, it is known to mount one or more strain gauges on the flow meter tube and to obtain a measure of strain from the strain gauges. Pursuant to the invention it is has been appreciated that provision of strain gauges may not give accurate results, as they only provide a local indication of strain. The strain gauges may also require calibration and temperature compensation.
Changes in tension are generally less of a problem in meters which have a higher compliance, for example where the length of the metering tube is large in comparison to the distance between fixing points and incorporates one or more bends, an example being a B-tube meter. Nonetheless, other factors which may affect stress in the tube, for example pressure, may affect density measurements or other measurements such as flow.
EP-A-848234 and U.S. Pat. No. 5,734,112 both disclose coriolis flow meters having two parallel bent tubes with sensors mounted close to first and second nodes of vibration and wherein the tubes are excited in two modes of vibration.
EP-A-701 107 discloses an arrangement in which the resonant frequencies of two vibration modes of a straight tube meter are measured and in which it is demonstrated that the ratio of the two frequencies is a linear function of tension. Thus, from the ratio of frequencies, a measure of tension can be calculated. We have investigated the techniques and assumptions proposed in that disclosure, as discussed further below and have discovered that, whilst the reasoning and results presented in that disclosure are useful and may provide a useful improvement on previous strain-gauge methods, there is room for improvement and the technique cannot produce highly accurate meters. Specifically, analysis pursuant to the invention reveals that, surprisingly, the relationship is not truly linear, nor can it be readily corrected, and better results can be achieved by a different approach. FIG. 15 shows the error in the stress estimate based on a technique as disclosed.
In a general aspect, the invention proposes use of measurements of resonant frequencies for two or more independent vibrational modes of a metering tube to obtain a measure of density of fluid in the metering tube compensated for variation of stress in the metering tube or to obtain a measure of stress in the metering tube wherein the ratio of said resonant frequencies is dependent on density or wherein the stress is determined as a non-linear function of the ratio of said resonant frequencies.
In a first method aspect, the invention provides a method of obtaining a measure of stress in a fluid metering tube, the fluid having a density, the method comprising inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density; obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and deriving said measure of stress from said first and second resonant frequencies based on modelling the fluid density as a first function of stress and the first resonant frequency and modelling the fluid density as a second function of stress and the second resonant frequency and solving to determine stress as a function of said frequencies.
In a second method aspect, the invention provides a method of obtaining a measure of stress in a fluid metering tube, the fluid having a density, the method comprising inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density; obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and deriving said measure of stress from said first and second resonant frequencies based on determining possible pairs of values of stress and density corresponding to one of the first and second resonant frequency and selecting a pair of values based on the other of the first and second resonant frequencies.
In a third method aspect the invention provides a method of obtaining a measure of density of fluid in a metering tube, the tube being subjected to a stress, the method comprising inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density; obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and deriving said measure of density from said first and second resonant frequencies based on modelling the fluid density as a first function of stress and the first resonant frequency and modelling the fluid density as a second function of stress and the second resonant frequency and solving to eliminate stress.
As will become apparent as the description proceeds, all of the above aspects stem from a common and novel approach to determination of density or stress in a metering tube which not only may provide better results than prior art techniques but may be simpler to implement. It will be apparent that the above techniques are not limited to implementations where the ratio of the two frequencies is independent of density; the methods may be used where the ratio of the first and second frequencies varies with density and/or where the stress is not a linear function of this ratio (whether or not this ratio is explicitly determined (unlike the prior art, there is no need to determine this ratio)). Whilst the invention works where such constraints are not met, it of course equally works if they are met; indeed such constraints are not directly relevant to the invention.
Preferred features are set out in the dependent claims and other preferable and optional features and the advantages thereof will be apparent from the following.
The invention extends to apparatus arranged to perform a method according to any method aspect, which may comprise a signal processor of a flowmeter and may include said metering tube.
In a first apparatus aspect, the invention provides apparatus for obtaining a measure of stress in a fluid metering tube, the fluid having a density, the apparatus comprising:
exciter means for inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density;
frequency measurement means for obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and
processing means for deriving said measure of stress from said first and second resonant frequencies based on modelling the fluid density as a first function of stress and the first resonant frequency and modelling the fluid density as a second function of stress and the second resonant frequency and solving to determine stress as a function of said frequencies.
In a second apparatus aspect, the invention provides apparatus for obtaining a measure of stress in a fluid metering tube, the fluid having a density, the apparatus comprising:
exciter means for inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density;
frequency measurement means for obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and
processing means for deriving said measure of stress from said first and second resonant frequencies based on determining possible pairs of values of stress and density corresponding to one of the first and second resonant frequency and selecting a pair of values based on the other of the first and second resonant frequencies.
In a third apparatus aspect, the invention provides apparatus for obtaining a measure of stress in a fluid metering tube, the fluid having a density, the apparatus comprising:
exciter means for inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density;
frequency determining means for obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and
processing means for deriving said measure of stress from said first and second resonant frequencies based on determining possible pairs of values of stress and density corresponding to one of the first and second resonant frequency and selecting a pair of values based on the other of the first and second resonant frequencies.
The invention further provides a method of determining parameters, functions, look-up tables or coefficients for use in the preceding aspects and further provides look-up tables and the like so generated. In an exemplary aspect of this, the invention provides a method for use in determining a measure of stress or density comprising performing a method according to any preceding method aspect for a plurality of values of stress and density and determining an empirical function or look-up table of values relating said first and second resonant frequencies to stress or density. The method may further comprise storing said empirical function or look-up table in memory means.
The invention further provides a computer program or computer program product comprising instructions for performing any methods disclosed herein.
Although the invention is primarily concerned with methods and apparatus as set out above, it is a further general aim of the invention to provide an improved method of obtaining a measure of a variable, particularly stress, which is dependent on factors such as tension or pressure and indirectly dependent on temperature, in a flow meter tube which may affect measurement of a parameter such as density, or to provide an improved measure of density or other parameter which is less prone to errors due to changes in the variable. Further aspects and preferred features may achieve this, and may be applied to the preceding aspects, as will be set out below.
The invention may provide a method of obtaining a measure of a variable in a metering tube comprising inducing a first vibration mode in the tube and obtaining a first measure of at least one characteristic of the first vibration mode which is a first function of the variable; obtaining a second measure of at least one second characteristic which is a second function of the variable; and deriving said measure of the variable from said first and second measures. The variable is preferably related to stress in the tube and may be a measure of tension or pressure or of stress itself.
The measure of the variable may be output directly. For example, it may be useful to obtain a measure of pressure in the tube. Alternatively, the variable may be used to derive a more accurate measure of density or other desired parameters of fluid in the tube. The variable need not be explicitly determined, but instead a measure of density or other desired parameter which is substantially independent of the variable may be derived directly from the first and second measures.
Thus, in a further aspect, the invention may provide a method of obtaining a measure of a desired parameter of fluid, preferably density of fluid, in a metering tube, the method comprising inducing a first vibration mode in the tube and obtaining a first measure of at least one characteristic of the first vibration mode which is a first function of said desired parameter and a further variable; obtaining a second measure of at least one second characteristic which is a second function of at least said further variable; and deriving a compensated estimate of said desired parameter from said first and second measures which compensated estimate of the desired parameter is substantially independent of the variable. By substantially independent is meant that the compensated estimate preferably has less dependence on the further variable than an estimate based on the first measure alone, although some dependence may remain. The variable is preferably related to stress in the tube and may be a measure of tension or pressure or of stress itself.
In both the immediately preceding aspects, the variable is determined (in the first aspect) or compensated for (in the second aspect) based on a measure of the first characteristic which is directly derived from the vibration induced in the tube. This may offer advantages over conventional methods which rely on indirect methods such as using strain gauges as the measurement should be directly affected in a similar way to desired properties which are being measured.
Although the primary effect of stress in a straight tube meter is on the apparent measurement of density, there is also an effect on other parameters such as flow rate. Although the effect on flow rate is generally smaller (because the flow rate may depend on a ratio of two frequencies, so similar errors in both frequencies will tend to cancel), it may nonetheless be desirable to correct any such errors and this may be made possible if the stress is known.
Although the second characteristic could in principle be any physical characteristic affected by the first variable (preferably stress-related), for example speed of sound or electrical conductivity, preferably said at least one second characteristic is at least one characteristic of a second vibration mode induced in the tube. The first vibration mode is preferably us ed to determine fluid flow characteristics, particularly density from the resonant frequency. By using one or more characteristics of a second vibration mode, the effects of other factors which tend to affect both measurements similarly may be minimised or cancelled and a more reliable determination of the overall effect of the second variable can be determined.
Preferably the first measure includes the resonant frequency of the first vibration mode and the second measure includes the resonant frequency of the second vibration mode. Both frequencies are functions of both the (stress-related) variable (primarily tension in a straight tube meter, primarily pressure in a thin-walled high compliance meter) in the tube and fluid density (and possibly other parameters). The measurements obtained may be combined to obtain a measure of density which is substantially independent of the tension in the tube. The measure of tension (or other variable) or density (or other parameter) may be derived from the measured resonant frequency according to a predetermined formula based on the dimensions and material of the tube, and optionally including a compensation factor to take into account the presence of fluid in the tube. Alternatively, the measure of tension (or other variable) or density (or other parameter) may be obtained from an empirically stored relationship between measured frequency and tension. This relationship may be derived by obtaining measurements from the tube, or from a similar tube during an initial calibration step.
The first and second vibration modes are preferably substantially orthogonal. The modes may be spatially oriented in substantially orthogonal respective first and second spatial directions, for example along respective x and y axes of the tube. Alternatively, one mode may be a longitudinal bending mode and the other mode may be a torsional vibrational mode. The tube may be selected or mounted to have different vibrational properties in said first and second spatial directions. For example, rather than a cylindrical tube, the tube may be asymmetric, preferably having a rectangular or oval cross-section; this will give a greater bending stiffness in the wider cross-sectional direction. Additionally or alternatively, the tube may be mounted to enhance or decrease bending stiffness in one of said spatial directions, for example by fixing on two opposite sides, but not around the whole perimeter of the tube; this will give greater bending stiffness in a plane passing through the fixing points (the use of an asymmetric tube to induce bending preferentially in one direction is known, but it has not been proposed to excite such a tube independently in two directions for the reasons outlined above). This may be used to enhance separation of vibration frequencies. Furthermore, by using spatially perpendicular modes, the sensors and actuators may be designed to couple primarily to the desired spatial mode and to be relatively poorly coupled to the other mode, thereby facilitating separation of signals and driving. Alternatively the modes may be orthogonal by virtue of frequency but be spatially coincident; this reduces the number of physical sensors required. Both modes may be longitudinal modes. The modes will normally have different resonant frequencies but this is not necessary. It is desirable, however, that the frequencies of the modes chosen are different functions of stress and density, for example that the variation with or partial derivative with respect to each of stress and density has a different value.
Surprisingly, we have found that measurement of a torsional vibrational resonant frequency may provide a convenient yet effective and reliable means for determining or compensating for tension within the tube, and may not be adversely affected by temperature; measurement of a torsional resonant frequency may be provided independently. A further advantage is that it may be possible to detect the vibration using the sensors, or modified sensors, used to detect the main (bending vibration mode) used to detect flow. Surprisingly, the torsional resonant frequency is normally largely unaffected by the flow of fluid in the tube, although it has been found that the viscosity of the fluid may affect the measurement. However, in many configurations, the torsional resonant frequency is only slightly dependent on stress. The method may include signalling a fault based on the measured value of the variable or based on the torsional resonant frequency. In particular, in the event of two values of the characteristic giving implausible values for stress or density, this may be used to signify a calibration drift or other possible fault. More preferably, three or more independent parameters, for example two vibration characteristics and an independent stress or temperature measurement, or three vibration characteristics may be compared to determine a measure of accuracy or to identify a potential fault.
The invention extends to apparatus for implementing any of the above method aspects.